Hi Raymond:
>Does anyone know why the standard selects 25A, not others? The presumption is that a zero-impedance fault occurs between the live mains and the protective earth/ground circuit. This means the full current of the mains will flow in the protective earth/ground circuit until the overcurrent device operates. The value of 25 amperes (for 1 minute) approximates the worst-case current and duration for operation of a mains overcurrent device such as a fuse or circuit-breaker. For example, fuses are required to operate in no more than one minute at twice rated current. So, if the mains circuit is protected with a 15-amp fuse, the fault current would be 30 amps for no more than 1 minute. (This is why CSA requires a test at 30 amps.) Circuit-breakers are required to operate in no more than four minutes at twice rated current. It is generally accepted (and it is largely true) that if the circuit can withstand 25 amperes for one minute, it can withstand 25 amperes or more for several minutes. Best regards, Rich ------------------------------------------------------------- Richard Nute Product Safety Engineer Hewlett-Packard Company Product Regulations Group AiO Division Tel : +1 619 655 3329 16399 West Bernardo Drive FAX : +1 619 655 4979 San Diego, California 92127 e-mail: ri...@sdd.hp.com ------------------------------------------------------------- --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to ed.pr...@cubic.com, j...@gwmail.monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators).
